This invention relates to electromagnetic thickness measurements in general, and more particularly to an improved method and apparatus for measuring the thickness of metal sheets.
It is well known in the art that the phase of an electric field resulting from eddy currents induced in a conducting sheet by a transmitter will have a phase relationship to the phase of the transmitter which is a function of the conductivity and the thickness of the conducting sheet. Various thickness gauges have been developed based on this principle. The simplest gauges developed for this purpose suffered from two deficiencies. First, they were only operable with sheets of known conductivity. That is, since the phase shift is a function of both conductivity and thickness, conductivity needed to be known in order to relate thickness and phase shift. Secondly, these gauges were unduly sensitive to the distance of transmitting and receiving coils from the conducting sheet being gauged. That is, the phase angle varied with the distance. Thus, they were impractical for measuring thickness of sheet metal as it was being manufactured because of the considerable transverse movement of the metal sheet making it impossible to maintain constant distances between the sheet and transmitting and receiving coils. Furthermore, in this type of application, the conductivity of the sheet tends to vary thereby further adding to the difficulty of making measurements.
An apparatus and method for solving this problem is disclosed in U.S. Pat. No. 3,764,897. In the electromagnetic thickness gauge disclosed therein, dependence on the distance of the coil from the sheet being gauged is eliminated through the use of a transmitting coil designed so as to generate a magnetic field so shaped that the phase angle of the sensed field due to the induced eddy current is substantially invarient with the distance between the metal sheet and the coil. To overcome the problem of conductivity, two eddy current measurements in which the phase angle varies in a different manner with variation in thickness and conductivity are taken. Thus, by solving simultaneous equations relating to these two phase angles, it is possible to determine both thickness and conductivity. A number of embodiments are shown, each in which a first measurement is made at a relatively low frequency where the phase angle is proportional to the inverse of the product of the conductivity and thickness with a second measurement made at a high frequency which the phase angle proportional to the square root of the conductivity. In each of the embodiments disclosed and illustrated, at least one measurement is a reflectance measurement.
Although the method and apparatus described therein works quite well, it suffers from a number of deficiencies which although not fundamental to its operations are of great practical importance. In particular, the signal to noise ratio required is very difficult to achieve in view of the following:
a. the necessity for balancing out signals from the transmitter; PA1 b. the requirements for high values of q to obtain adequate phase shift; PA1 c. the requirement for sufficient separation between the sheet and transducer to accomodate normal flutter and other tolerances in the sheet position. Furthermore, a specially designed coil of complex construction is required. In view of this, it is clear that there is need for an improved method and apparatus for making thickness measurement which does not suffer from these deficiencies. PA1 a. the signal to noise ratio is very good and is not subjected to the requirement of bridge balancing; PA1 b. since the distance between the transmitter and receiver coils is constant, a design providing a constant mode parameter q over a range of separations is not required and simple coils can be used; PA1 c. the sensitivity in terms of the size of the angle sensed is good; PA1 d. measurements may be taken at lower frequencies; and PA1 e. the method may be carried out with considerably lower values of q than are necessary for making reflectance measurements and as a result much larger coils spaced further apart can be used.